Re: RTL PUA?

John Hudson 於 2011年8月23日 下午9:08 寫道：

> I think you may be right that quite a lot of existing OTL functionality wouldn't be affected by applying BiDi after glyph shaping: logical order and resolved order are often identical in terms of GSUB input. But it is in the cases where they are not identical that there needs to be a clearly defined and standard way to do things on which font developers can rely. [A parallel is canonical combining class ordering and GPOS mark positioning: there are huge numbers of instances, even for quite complicated combinations of base plus multiple marks, in which it really doesn't matter what order the marks are in for the typeform to display correctly; but there are some instances in which you absolutely need to have a particular mark sequence.]

And this is really the key point. There really isn't anything inherent to OpenType that absolutely *requires* the bidi algorithm be run in character space. It would theoretically be possible to manage things in a fashion so that it's run afterwards, à la AAT. But font designers *must* know which way it's being done in practice, and, in practice, all OT engines run the bidi algorithm in character space and not in glyph space. At this point, trying to arrange things so that it can be done in glyph space instead is a practical impossibility.

=====
Hoani H. Tinikini
John H. Jenkins
jenkins_at_apple.com
Received on Wed Aug 24 2011 - 09:49:28 CDT